It is well known in the glass art that glass is quite strong under compressive stress, and that, ordinarily, glass breakage occurs as the result of tensile stress being applied to some portion of the glass article. It is primarily under tensile stress that surface cracks or defects propagate readily into the interior of the article to cause breakage. Most well-known glass-strengthening methods take advantage of this characteristic by providing a compressively-stressed surface layer on the glass article to be strenghtened. The compressive stress inhibits the propagation of surface cracks into the article and must be overcome by large counterbalancing tensile stresses before crack propagation and failure will occur.
Recent methods of providing such compressively-stressed surface layers include ion-exchange strengthening. Ion exchange methods generally involve contacting the surface of the glass to be strengthened with a source of exchangeable ions at elevated temperatures to cause the migration of ions from that source into the glass in exchange for certain ions from the glass. In this way, the composition of the surface layer of glass is changed so that a compressively-stressed region is formed which adds considerably to the strength of the article. Such a procedure suffers from several major disadvantages. It usually involves the use of molten salts at elevated temperatures to provide a source of exchangeable ions, and is, therefore, hazardous and expensive. It also provides a rather thin compressively-stressed surface layer on the treated glass which is subject to penetration by simple abrasion is everyday use, thus rendering such articles mechanically unstable. Finally, the strength provided by such a layer is subject to degradation at elevated temperatures due to the release of the compressive stress as the strain point of the glass is reached.
Similar ion-exchange strengthening procedures have more recently been discovered for semicrystalline glasses or glassceramics, which are produced through the crystallization in situ of certain glasses containing crystallization-promoting or nucleating agents by the use of an appropriate heat treatment. However, the use of glass-ceramics has not overcome the inherent disadvantages of ion-exchange strengthening procedures.
Even more recently, commercially practicable methods have been discovered for producing laminated glass and glass-ceramic articles having compressively-stressed surface layers as the result of a thermal expansion mismatch between the core and surface layer glasses. U.S. Pat. No. 3,673,049 issued to Giffen et al. describes a method of forming a glass laminate at elevated temperatures so as to obtain intimate bonding or fusion of the core and adhered surface layers and thus, an interface which is essentially defect-free. Such a laminate depends for its strength upon a difference in thermal expansion between the core and surface glasses over the temperature range from about the setting point of the laminate to room temperature, such that the core will contract more than the surface layers upon cooling. In this way, a laminate having compressively-stressed surface layers and a tensilely-stressed core at room temperature is provided. Such articles are efficiently manufactured from relatively inexpensive starting materials, but since they generally incorporate at least one high expansion material to provide a sufficient expansion mismatch, their thermal shock resistance is not good. Furthermore, the strength provided by the compressively-stressed surface layer at room temperature is lost at elevated temperatures due to the same expansion mismatch, which causes a reversal of the stress buildup upon heating, so that such articles do not offer good high temperature strength. The use of low-expansion thermally-crystallizable glasses or glass-ceramics in forming the laminate, although somewhat helpful in improving thermal shock resistance, does not avoid the loss of strength at elevated temperatures which occurs as the inevitable result of the presence of a substantial thermal expansion mismatch.
It is accordingly the principal object of the present invention to provide strengthened glass-ceramic articles offering not only high strength, mechanical stability and good thermal shock resistance at room temperature, but also excellent strength retention at high temperatures.
It is a further object of the present invention to provide a method of making such articles which can be employed simply and economically using a wide variety of glass-ceramics of differing chemical and physical properties to provide strengthened products having desirable characteristics.
Other objects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, and from the appended drawings showing in schematic form several embodiments of the mechanism by which strengthening is achieved according to the present invention.